This invention relates to AC machines and more particularly to slot wedges therefor, or top sticks as they are sometimes called.
In an AC machine having a toothed stator, slot wedges are used to hold the stator windings in the slots formed between the stator teeth. Slot wedges made of magnetic material are a significant means to improve the efficiency of an ac motor. Magnetic slot wedges reduce slot ripple in the air gap flux caused by the changing reluctance due to the slots, and also reduce the associated eddy current losses due to the interaction of the harmonics in the air gap flux with the conducting surface of the rotor. The magnetizing current required in the stator windings to generate the desired air gap flux is less with magnetic slot wedges, since more of the air gap flux is available for useful power production. However, closing the slot completely with magnetic material increases the leakage reactance of the motor, which in the case of an induction motor results in a reduction of power factor and of peak torque, and in a synchronous motor a reduction of peak torque and slower dynamic response.
The present methods of making magnetic slot wedges with wire or iron powder embedded in a carrier, generally do not allow easy shaping of the magnetic material in a way which reduces slotting harmonics or, as they are sometimes called, space harmonics, with a minimum increase in slot leakage reactance. Another problem is that attempts to manufacture magnetic slot wedges that are structurally sound and do not fail during operation have only been partially successful. Nonmagnetic slot wedges do not have the failure problems that magnetic slot wedges have. Thus, it is expected that the different magnetic forces, the different loss characteristics and the different thermal characteristics of the magnetic slot wedges are responsible for their limited life in actual operation.
To achieve reduced slot harmonic losses, stators with semiclosed slots have been used. Semiclosed slots, as the name implies, provide a narrow opening at the top of the slot and require random windings. Random windings are windings in which the relative position of one wire to another is not known until the wires are pushed through the narrow opening and pressed into the slot. Formed coil windings cannot be inserted into semiclosed slots. Formed coils are used for high voltage (above 600 volts) applications because of their superior turn and ground insulation properties and for larger machines (above 600 HP) because of their superior reliability, heat transfer capability and easy manufacturability in the larger coil sizes. When open slots are used in conjunction with magnetic slot wedges, formed coils can be used without sacrificing the advantages of semiclosed slots. In a formed coil the windings are preformed and the position of each wire relative to each of the other wires is known prior to insertion in the slot.
It is an object of the present invention to provide a magnetic slot wedge that results in reduced slot harmonic losses and sufficient strength to carry all the forces the wedge is exposed to.
It is a further object of the present invention to provide a magnetic slot wedge that results in reduced space harmonic losses and permits formed windings to be used.
It is a still further object of the present invention to provide a magnetic slot wedge which reduces the slotting harmonics with a minimum increase in slot leakage reactance.